The present invention relates to nuclear reactors and more particularly to a control system for a liquid metal-cooled fast breeder reactor.
A nuclear reactor of the character described is a dual purpose reactor in that it produces nuclear fuel while generating useful energy. The useful energy, which initially is in the form of heat, is produced primarily by fission of plutonium 239 and uranium 238. This fission process produces neutrons in excess of those needed to sustain a nuclear chain reaction. On capture of an excess neutron, a fertile isotope such as uranium 238 is converted to a fissile isotope plutonium 239. The amount of fissile plutonium 239 thus produced actually exceeds the amount of plutonium consumed; hence, the reactor system is entitled "a breeder reactor." Further, since the conversion process is optimal at high neutron speeds, the reactor title is modified to include the word "fast." The fissile plutonium 239, the fertile uranium 238 and some minor amounts of other plutonium and uranium isotopes are combined within fuel rods which in turn are assembled in fuel assemblies. A nuclear core of the reactor is comprised of a plurality of such fuel assemblies arranged in a manner consistent with overall nuclear efficiency.
The heat generated by the fission process within the fuel assemblies is removed by the passage of a reactor coolant through the nuclear core. In a nuclear reactor of the type described herein, the reactor coolant must not slow down the fast neutrons emitted by the fission process because optimal conversion of the fertile uranium isotope requires neutrons having energy levels greater than 0.1 MEV. That is, the reactor coolant must not operate as a moderator as it does in pressurized water reactors; yet, the reactor coolant must possess good heat transfer characteristics so as to efficiently remove the heat from the nuclear core. Sodium, heated to the liquid state, is an example of a reactor coolant satisfying the requirements of a fast breeder reactor.
Typically, a reactor is capable of producing more neutrons than required to sustain criticality at its design power level. This capability is advantageously used during reactor start up and increasing its power to design levels. Once the design power level has been reached however, it is necessary to decrease the availability of neutrons in order to maintain the reactor at steady state operation. The adjustment of the availability of neutrons in a fast breeder nuclear reactor is achieved by insertion or withdrawal of control rods comprised of boron carbide, tantalum or some other suitable neutron absorbing material. These solid materials have high neutron absorbing characteristics which is commonly referred to as large neutron capture cross section. Thus, by varying the position of the control rods, either more or less neutron absorbing material is exposed to the nuclear core and the rate of nuclear fission changes accordingly. Rapid insertion of a number of control rods, generally known as scramming, or rapidly deactivating the reactor, serves to decrease quickly the rate of nuclear fission of the nuclear core and shuts down the reactor.
Heretofore, reactors of the character described have been equipped with a secondary safety system comprising an additional control rod system. The single purpose of this additional control system is to permit reactor shut down should the primary control rods become inoperative. Quite often, the secondary safety system, like the primary safety system, comprises control rods containing solid neutron absorbing materials. With both sets of control rods being similar, it is highly probable that the secondary safety system would become inoperative for the very same reasons that caused failure of the primary control system. The result is that no backup safety system actually exists.
Other backup safety systems, in prior reactors, have attempted to overcome this stuck or inoperative rod problem by utilizing a liquid control rod system, that is, a control system comprising a liquid neutron absorber. In the prior art, these control systems were characteristically of two types. The first type injects the neutron absorber liquid directly into the reactor coolant. Although effective, this method suffers from a relatively long response time because of the large volume of neutron absorber fluid required to attain sufficient concentration in the reactor coolant to cause reactor shutdown. Such a method further suffers from the requirement to separate, rapidly and economically, the neutron absorber fluid from the reactor coolant once the backup safety system has been used. Traditionally, cleanup of the reactor coolant has neither been rapid nor economical. The second type of liquid control rod system comprises closed flow paths or tubes placed within the reactor core through which the liquid neutron absorber flows when the safety system is activated. While this technique eliminates the mixture problems inherent in the first type of backup control system, it is extremely disadvantageous in that it occupies core space which would otherwise be used for fuel. Thus, nuclear efficiency of the reactor is decreased and fuel cycle costs are increased.